The objectives of this research proposal is to participate as a clinical center in a multicenter trial to determine whether the development of non- insulin dependent diabetes (NIDDM) can be prevented. We propose to identify 200 subjects at high risk for developing NIDDM over a 12-month period. Group I subjects with impaired glucose tolerance (IGT) will be identified from the African-American minority group by oral glucose tolerance testing (OGTT) and Group Il subjects with IGT will be identified from a large data base of women who have had post-gestational diabetes. In the course of screening these populations, it is anticipated that a proportion of subjects with undiagnosed diabetes will be identified by OGTT. Those without fasting hyperglycemia (<140 mg/dl) represent a very high risk group for progression to fasting hyperglycemia and will be included in the trial (Group III). Recruited subjects will be randomized either into a usual referred care (RC) group and a special intervention (SI) group aimed at achieving dietary modification (4% weight loss) and increased physical activity (600 Kcal/week) for a 4-5 year period. Half of the SI group will in addition receive the antihyperglycemic drug, Metformin (850 mg b.i.d.), and the other half placebo. Performance of yearly OGTT's will be used to compare rates of deterioration to fasting hyperglycemia in each of the three treatment groups, in addition to changes in insulin and cardiovascular risk factors. Subjects with IGT will be analyzed separately from those originally found to have diabetes, since the natural history and response to treatment may differ between the two. It is anticipated that intervention in patients at high risk for diabetes, with diet, physical activity or pharmacologic agents may slow or prevent their deterioration to NIDDM and offer a new approach to the treatment of this disease.